ARIZONA WATER COMPANY

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Calvin Phelps
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1 ARIZONA WATER COMPANY East Sedona Water Storage Facility February 21, 2018 Community Meeting

2 STORMWATER RUNOFF

3 Issue: Runoff sometimes flows out of the wash at bottlenecks in front of several homes on W. Mallard Questions: How will Arizona Water s project affect runoff? How is stormwater runoff calculated? 3

4 s Existing Site 1.05 Acres (45,738 sq. ft.) Two lots Topsoil is 2 to 3.5 ft. +/- deep Sloped Site drains to North and Southwest 4

5 Sedona Routinely Handles Drainage Issues City s Land Development Code includes a well-defined formula for calculating runoff and managing flood risk. Rational Drainage Equation: Q (cfs) = C x I x A Q C I A Quantity of stormwater runoff, in cubic feet per second (cfs) Runoff Coefficient determines the amount of rainfall that is NOT absorbed by the ground Precipitation. Drainage Design Table 8.1 Applicable Sections Area 5

6 Determining the Coefficient: Start with Type of Soil Q=C x I x A Four factors as defined by the U. S. Department of Agriculture, Hydrologic Soils Group A - Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep and well to excessively drained sands and gravels. These soils have a high rate of water transmission. B - Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission. C - Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine texture. These soils have a slow rate of water transmission. D - Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan at or near the surface, and shallow soils over nearly impervious material. These soils have a very slow rate of water transmission. 6

7 Site Analysis Q = C x I x A 7

8 Site Analysis Q = C x I x A 8

9 Precipitation and Drainage Design Tables 8.2, 8.3 Q = C x I x A Tables copied from Sedona Land Development Code Table 8.3 Upper Limit Precipitation Frequency Estimates Table 8.2 Mean Precipitation Frequency Estimates Freq hr 6-hr 12-hr 24-hr (yr) min min min min min min day day day day day Freq hr 6-hr 12-hr 24-hr (yr) min min min min min min day day day day day , ,

10 Storm Frequency: 25-yr. Froude No.: FN 0.86; 1.13 FN 2.0 Freeboard (Minimum): Subcritical Flow Supercritical Flow Maintenance Road: Maximum Velocity: 100-yr. (check) FB = 0.25*[y+(v 2 /2g)] (1-ft. min.) FB = 0.25*[y+(v 2 /2g)] (2-ft. min.) 12 ft. wide, 1 side of channel per channel lining material Sedona Routinely Handles Runoff and Flooding Issues 4.0 Open Channels Maximum Side Slope: Vegetal/Earth (ss = from slope stability analysis) 3:1 or ss Loose Riprap Rigid Lining Shotcrete 1:1 3:1 or ss ss Q = C x I x A Soil Cement 1:1 Building Setback from Channel Bank and Floodway: 1.5 x the channel depth, unless otherwise approved by the City Engineer for engineering reasons. However, in any case, the more restrictive requirements within this article shall apply. Storm Frequency: 2-, 10-, 25-, and 100-yr. storms Criteria: post-proj. Q pre-proj. Q 5.0 Stormwater Storage When Required: Maintenance Road: Maximum Depth: Parking Areas Emergency Spillway: 1 acre or larger development or when postdevelopment flow will exceed pre-development flow by 1 cfs. 12-ft. access road pass post-developed 100-yr. Q Table Copied from Sedona Land Development Code Maximum Drain Time: 12 hrs. upstream watershed areas 10 ac. and 24 hrs. for an upstream watershed area > 10 ac. Freeboard (Minimum): 1 ft. (post developed 100-yr. event) Min. Principal Outlet: 12 inch Maximum Side Slopes: Depth < 3 ft. 2:1, protected; 3:1, unprotected 10 Depth 3 ft. 4:1

11 Runoff Calculations 100 Yr- 1 Hr Storm Rainfall Depth, inch Q = C x I x A Yavapai County Drainage Policies & Standards V=C(P/12)A 2.84 Area, sqft City of Sedona Precipitation Frequency Estimates Existing Area, acres C - Value Peak Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description Existing Grass and Brush - Area 1 4, Grass and Brush - Area 2 41, ,381 Pavement & Rooftops Roadway/Concrete Pads Reservoir Reservoir 10 ft backfill ring Total 45, ,954 Area, sqft Future Area, acres C - Value Peak Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description - Proposed Grass and Brush - Area 1 4, Grass and Brush - Area 2 20, ,650 Hillslope Sonoran Desert - Landscaped 2, area above reservoir Pavement & Rooftops Roadway/Concrete Pads Reservoir 7, ,701 Reservoir 10 ft backfill ring 10, ,254 Total 45, ,622 Excess Runoff - Area 1 (Retention Volume Required, cft) Excess Runoff - Area 2 (Retention Volume Required, cft) Excess Runoff (Retention Volume Required, cft)

12 Runoff Coefficient Sensitivity Analysis Runoff increase still < 1 cfs of existing conditions Yavapai County Drainage Policies & Standards V=C(P/12)A 100 Yr- 1 Hr Storm Rainfall Depth, inch 2.84 Sedona Precipitation Frequency Estimates V=C(P/12) Yavapai County Drainage Policies & Standards A 100 Yr- 1 Hr Storm Rainfall Depth, inch 2.84 Sedona Precipitation Frequency Estimates Existing Area, sqft Area, acres C - Value Peak Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description - Existing Grass and Brush - Area 1 4, Grass and Brush - Area 2 41, ,848 Pavement & Rooftopes Roadway/Concrete Pads Reservoir Reservoir 10 ft backfill ring Total 45, ,577 Existing Area, sqft Peak Area, acres C - Value Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description - Existing Grass and Brush - Area 1 4, Grass and Brush - Area 2 41, ,446 Pavement & Rooftopes Roadway/Concrete Pads Reservoir Reservoir 10 ft backfill ring Total 45, ,706 Future Area, sqft Area, acres C - Value Peak Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description - Proposed Grass and Brush - Area 1 4, Grass and Brush - Area 2 20, ,372 Hillslope Sonoran Desert - Landscaped area above reservoir 2, Pavement & Rooftopes Roadway/Concrete Pads Reservoir 7, ,701 Reservoir 10 ft backfill ring 10, ,254 Total 45, ,601 Area, sqft Future Peak Area, acres C - Value Discharge, cfs Runoff Volume, acre-ft Runoff Volume, cft Description - Proposed Grass and Brush - Area 1 4, Grass and Brush - Area 2 20, ,204 Hillslope Sonoran Desert - Landscaped area above reservoir 2, Pavement & Rooftopes Roadway/Concrete Pads Reservoir 7, ,701 Reservoir 10 ft backfill ring 10, ,254 Total 45, ,662 Excess Runoff - Area 1 (Retention Volume Required, cft) Excess Runoff - Area 2 (Retention Volume Required, cft) Excess Runoff (Retention Volume Required, cft) Excess Runoff - Area 1 (Retention Volume Required, cft) Excess Runoff - Area 2 (Retention Volume Required, cft) Excess Runoff (Retention Volume Required, cft) 0 1,956 1,956 12

13 Project Conditions Retention 2 8 ft MH Existing Site 1.05 acres 113 ft. diameter tank 10 ft. ring around tank sand Building above tank 2,840 sq.ft. soil cover over tank Impermeable area 7,200 sq.ft. Retention 1 13

14 Retention Basin Calculations 1 hr. Rain Duration Storm Frequency Match Existing Cndts Retention Proposed Retention 1 Proposed Retention Storm Water Flow w Proposed Retention Inch Exist Project % in 1 hr Cndts Cndts Increase Required 2 Total Reduction (yr) (cfs) (cfs) (cfs) (cft) (cft) (cft) (cft) (cft) (cfs) % % % % % Retention 1 located along the southwest corner of the site Retention 2 8 ft. manhole located north of the proposed tank 14

15 Retention Basin Calculations 2 hr. Rain Duration Storm Frequency Match Existing Cdts Retention Proposed Retention 1 Proposed Retention 2 Storm Water Flow w Proposed Retention Inch in 2 hrs Exist Cndts Project Cndts Increase Required Total Retention % Reduction (yr) (cfs) (cfs) (cfs) (cft) (cft) (cft) (cft) (cft) (cfs) % % % % % Retention 1 located along the southwest corner of the site Retention 2 8 ft. manhole located north of the proposed tank 15

16 WATERSHED DISCUSSSION

17 Sedona Watersheds Culvert 17

18 Sedona Watersheds Culvert 18

19 Wash Culvert Project Site Culvert Culvert 2 30/36 Culverts Wall 2 Culverts of various sizes 19

On Site Runoff Compared to Wash Flows Two culverts convey storm

obtained from ADOT drawings: Culvert 100 box culvert 100 yr.

storm flow 190 cfs Culvert 109 54 pipe 100 yr.

20 On Site Runoff Compared to Wash Flows Two culverts convey storm water runoff from east to west of SR 179 Culvert information obtained from ADOT drawings: Culvert 100 box culvert 100 yr. storm flow 240 cfs 50 yr. storm flow 190 cfs Culvert pipe 100 yr. storm flow 68 cfs 50 yr. storm flow 55 cfs Additional drainage from SR 179 In comparison, site runoff is less than 1% of flow in wash 20

21 Unnamed Wash Floodplain Analysis USACE HEC-RAS floodplain modeling software Existing and proposed conditions 100-year (standard) and 50-year storm events, Discharges from ADOT plans No ineffective flow areas, no obstructions, and no structures Manning s roughness values main channel overbanks Culvert Wash 2 30/36 culverts 2 culverts of various sizes Project Site Culvert Wall Subcritical flow regime Standard for natural watercourses Downstream boundary condition only 21

natural flowpath Modeled as a combination of culverts and

22 Floodplain Analysis: Boundary Condition Known water surface elevation Existing culverts and driveway impede natural flowpath Modeled as a combination of culverts and weir CulvertMaster FlowMaster Resulted in ~5.7 of flow depth WSE 22

Floodplain Modeling Results: Cross-Sections @ Downstream

23 Floodplain Modeling Results: Downstream Proposed Tank Proposed Retaining Wall and Grading 23

24 Floodplain Modeling Results: Flowline Profile 24

25 Floodplain Modeling Results: WSEs 25

26 Floodplain Modeling Results: Map 26

Pollutant Removal Benefits

Pollutant Removal Benefits Bioswales Bioswales Similar to biocells, but have a slight, but positive grade toward an outlet Designed to convey the WQv event at very low velocities Promote filtration through native vegetation, infiltration